The longest I've allowed a Dweller to explore the Wasteland is 12 hours and they're still only finding BB Guns and other useless junk. So, how do you usually equip your explorers, how many do you send out at once, how long do you let them roam the Wastes, and what are their stats?
As pointed at in this question here, the luck of your wanderers has an impact on what they will find. Honestly from observations luck has More of an impact than time to get nice gear but time is still a factor to get Great gear.
My longest runs so far are around 36 hours of exploration and 18 hours return trip.
The low level luck people I used to send out would still bring back rusty whatever guns and BB guns even towards the end of their trips. They did some times come back with a nice rifle or the like but that was rare.
The high level luck people I have started using are amazing. They still find rusty guns and BB guns but they are also finding rail guns and power armor. This is making me definitely focus more on luck for pretty much everyone in the game.
To answer the general questions that followed, I send between 1 and 6 people out at once, I try to make their trips last as long as I can by loading them down with Stimpacks and RadAways. For stats I focus on Strength, Perception, Endurance and Luck. These stats appear to help them last longer and fear encounters less as well give an upturn as to what items they are bringing back with them.
NOTE: The -huge- majority of my guys WILL die before I bring them back as I can not check in on them as often as I should. Every time the amount of caps and gear on them greatly out-weights the cost of the revival (as its over 1000 now for them).